Set rc_dir based on W3M_DIR environment variable.

By default, w3m puts all of its data in the `~/.w3m/` directory
(creating it as necessary). This was not configurable in any way.

This commit adds some quick reconfigurability -- when the "W3M_DIR"
environment variable is set, w3m will use that location instead. The
default location is unchanged.

Fixes #130.

doc/MANUAL.html

@@ -91,6 +91,11 @@ insertions tagged with "mh 2016-03-29" and "mh 2016-06-11" come from the latest
    from standard input and display it. If it doesn't find a document
    there either then normally w3m will terminate.
   </p>
+  <p>
+   You can change how w3m behaves by providing it with options, either
+   via the command line or through the configuration file. The user
+   config file is located at $W3M_DIR/config (~/.w3m/config by default).
+  </p>
   <p>
    Options include:
   </p>